The process for the electrolytic decomposition of aqueous hydrochloric acid by means of the diaphragm process was developed jointly by the former Hoechst AG and Bayer AG as well as thyssenkrupp Uhde Chlorine Engineers. This collaboration is now being continued between Covestro and thyssenkrupp Uhde Chlorine Engineers.
The process consists of hydrochloric acid recycles for anolyte and catholyte, an absorption unit, product treatment, and the electrolyzer. The cell elements consist of two graphite electrodes separated by either a PVC or PVC/PVDF diaphragm. The individual cell elements are assembled similar to a filter press type bipolar electrolyzer.
Hydrochloric acid is fed to the anode compartment; the chloride ions are oxidized at the anode while the hydrogen ions pass through the ion exchange membrane and migrate to the cathode chamber. The hydrochloric acid exits depleted from 23 to 17%. Hydrochloric acid is fed at the cathode. Chloride ions migrate to the anode chamber where chlorine builds up. At the cathode hydrogen is formed which leaves the cell. The hydrochloric acid exits depleted from 21 to 17%.
Chlorine gas is freed on the anode side and hydrogen on the cathode side. In order to improve gas discharge, the electrodes are provided with a large number of vertical slots. The individual electrodes are fitted into frames made of synthetic material resistant to hydrochloric acid and chlorine. The diaphragms are made of special PVC or PVC / PVDF cloth.